An electric power steering apparatus is a supporting apparatus that engages a motor to assist steering force when the driver operates the steering wheel while the automobile is moving. An electric power steering apparatus includes a steering torque sensor for sensing steering torque generated in a steering shaft by the handling operations of the driver. In an electric power steering apparatus, a steering torque signal outputted from the steering torque sensor and a vehicle speed signal outputted from a vehicle speed sensor are used to perform control by means of a motor drive controller and to use PWM to drive a supporting motor that outputs auxiliary steering force, thus reducing the steering force for the driver.
Known conventional examples of a steering torque sensor in an electric power steering apparatus include a torsion bar-type torque-sensing device provided between the input and output shafts of a pinion, and a magnetostrictive torque sensor device that uses magnetostrictive effects.
A known example of a sensor portion of a magnetostrictive torque sensor device is, e.g., a structure wherein magnetostrictive films of Ni—Fe plating are provided at two top and bottom locations on the surface of the steering shaft linked to the steering wheel. These two magnetostrictive films are provided with magnetic anisotropy in opposite directions and are formed over specific widths in the axial direction. When steering torque is applied to the steering shaft, the inverse magnetostrictive characteristics generated based on the magnetic anisotropy in these two magnetostrictive films are brought out via an exciting coil and a sensing coil disposed around the peripheries of the magnetostrictive films, and the steering torque is sensed.
With reference to FIG. 17, the essential configuration of the sensor portion of a conventional magnetostrictive torque sensor device will be described. The sensor portion will hereinafter be referred to as “magnetostrictive torque sensor.” The magnetostrictive torque sensor 300 includes one magnetostrictive film 302 formed around the periphery of a steering shaft 301, another magnetostrictive film 303 formed at a space from the magnetostrictive film 302, an exciting coil 304 disposed in proximity to the magnetostrictive films 302, 303 and separated by an extremely small gap, and detecting coils 306, 307 provided in a corresponding manner to the magnetostrictive films 302, 303, respectively. An exciting voltage supply source 305 is connected to the exciting coil 304.
In this magnetostrictive torque sensor 300, stress is created in the magnetostrictive films 302, 303 when steering torque acts around the axial center of the steering shaft 301. Magnetostrictive effects are generated in the magnetostrictive films 302, 303 in accordance with this stress. In view of this, AC exciting voltage is supplied from the exciting voltage supply source 305 to the exciting coil 304, and the change in magnetic field caused by the magnetostrictive effects of the magnetostrictive films 302, 303 is detected by the detecting coils 306, 307 as a change in impedance. The steering torque applied to the steering shaft 301 is sensed based on this change in impedance. Instead of detecting a change in impedance, another possibility is to detect a change in induced voltage. The following is a description of a case in which a change in impedance is detected.
FIG. 18 is a graph showing magnetostrictive characteristics. The horizontal axis represents steering torque applied to the steering shaft 301, and the vertical axis represents the change in impedance generated in the detecting coils 306, 307 when AC exciting voltage is applied to the exciting coil 304. The curve C110 represents the change in impedance brought out by the detecting coil 306, and the curve C11 represents the change in impedance brought out by the detecting coil 307.
In the detecting coil 306, the impedance increases as the steering torque goes from negative to positive, the impedance peaks when the steering torque reaches the positive value T1, and the impedance decreases when the steering torque is greater than T1. Conversely, in the detecting coil 307, the impedance peaks when the steering torque reaches the negative value −T1, and the impedance decreases when the absolute value of the steering torque increases.
As is clear in FIG. 18, the steering torque-impedance characteristics obtained in the detecting coil 306 and the steering torque-impedance characteristics obtained in the detecting coil 307 form substantially convex curves. The steering torque-impedance characteristics (curve C110) obtained in the detecting coil 306 and the steering torque-impedance characteristics (curve C111) obtained in the detecting coil 307 are also symmetrical about the vertical axis that passes through the point of intersection between the characteristic curves. This is because the two top and bottom locations of the magnetostrictive films are magnetically anisotropic in opposite directions.
The straight line L10 represents values obtained by subtracting the characteristic curve C111 detected by the detecting coil 307 from the characteristic curve C110 detected by the detecting coil 306. The value of the straight line L10 is 0 when the steering torque is 0. The magnetostrictive torque sensor 300 outputs a sensory signal corresponding to the direction and strength of the input torque by using the area assumed to have a substantially constant slope in the vicinity of the torque neutral point. The characteristics of the straight line L10 can be used to sense the steering torque from the values of the detecting coils 306, 307.
FIG. 19 shows a flowchart of a conventional method for manufacturing a magnetostrictive torque sensor. This conventional method for manufacturing a magnetostrictive torque sensor includes a step for processing a pinion at the bottom end of the steering shaft (step S101), a step for hardening the pinion (step S102), a step for tempering the pinion (step S103), a step for providing the magnetostrictive films (S104), a step for providing twisting torque (step S105), a step for heat-treating the magnetostrictive films (step S106), a step for cooling the magnetostrictive films (step S107), a step for canceling the twisting torque (step S108), and a step for performing another heat treatment (step S109). These steps are disclosed in, e.g., Japanese Patent No. 3730234. In cases in which two magnetostrictive films are provided as shown in FIG. 17, the films are provided with magnetic anisotropy in steps S105 through S108.
The method for providing magnetic anisotropy to the two magnetostrictive films 302, 303 is a method wherein the magnetostrictive films 302, 303 are heated by high-frequency heating while torque is applied to the steering shaft 301, causing the magnetostrictive films 302, 303 to creep, then torque remains applied while the steering shaft 301 and the magnetostrictive films 302, 303 are cooled to room temperature, after which the torque is canceled. The impedance value characteristics in relation to input steering torque in the detecting coil 306 form a substantially convex curve having a maximum value P1 while steering torque is positive, as indicated by the curve C110 shown in FIG. 18. The detecting coil 307 produces a substantially convex curve having a maximum value P2 while steering torque is negative, as indicated by the curve C111.
The values of steering torque that result in the maximum values of the curves C110 and C111 vary depending on the strength of the torque applied when anisotropy is provided. As the applied torque increases, so does the difference between the steering torques that result in the maximum values of the two curves C110 and C111. For example, when the applied torque is greater than the applied torque for obtaining the characteristic curves C110 and C111, the characteristic curves are the curve C210 and the curve C211. At this time, the impedance value detected by the detecting coil 306 increases as the steering torque increases, the impedance reaches a maximum P12 when the steering torque reaches a positive value T2, and the impedance decreases when the steering torque is greater than T2. The impedance value detected by the detecting coil 307 reaches a maximum P22 when the steering torque reaches a negative value −T2, and the impedance decreases when the absolute value of the steering torque further increases. The straight line L20 then represents the values obtained by subtracting the characteristic curve C211 detected by the detecting coil 307 from the characteristic curve C210 detected by the detecting coil 306, and the slope of this straight line is less than that of the straight line L10. The reason for this is that the impedance characteristics have a smaller slope at the base and less linearity. The maximum values P12 and P22 have a smaller slope and less linearity than the maximum values P1 and P2 at the neutral position, as shown in FIG. 18. Therefore, when the torque applied in the presence of anisotropy is increased, the slope of impedance outputted in relation to torque inputted to the steering shaft is smaller than when less torque is applied. Thus, the gain expressed by the ratio between output values and input values decreases as the torque applied in the presence of anisotropy increases, as shown in FIG. 20. When the heating temperature differs, the amount of anisotropy varies as does the state of creeping in the magnetostrictive films, and the gain varies depending on the heating temperature as shown in FIG. 21. However, it is difficult in the manufacturing steps to manage all heating temperatures and torques applied to the steering shaft in the presence of anisotropy, and manufacturing has been complicated. It has thereby been made difficult to widely provide torque sensors that are highly rigid, have a high degree of linearity, and have small uniformities among individual sensors. Therefore, in the manufacture of magnetostrictive torque sensor devices including the magnetostrictive torque sensor described above, there is a demand for a method that would reduce nonuniformities in the gain of magnetostrictive torque sensor devices as finished products.